The purpose is to determine which aspects of speech production timing are independently controlled by the central nervous system and the structures controlling them. Patients with neurological disorders/diseases affecting a particular brain region are examined on experimental tasks manipulating separate aspects of speech timing including: syllable initiation time; syllable execution time; alteration of speech in sentences and syllable repetition rate. A series of studies is ongoing which demonstrate that although these aspects of speech timing are inter-related in normal speakers, they are independently affected in different neurological diseases. The results suggest that change in execution time is affected in Huntington's disease while change in speaking rate is affected in Parkinson's disease. Syllable initiation time was most affected when the white matter tracts were involved. Perceptual studies are employed to evaluate the significance of different speech production disorders for intelligibility. Patients' productions of p, b, and g are identified by listeners in a forced choice procedure. Acoustic measures of articulator timing are compared between correctly identified and incorrectly identified patient productions to determine which articulatory errors result in poor intelligibility in dysarthric speech. Single syllable productions of Parkinson patients were highly intelligible with few listener errors although there were significant acoustic differences between patients and controls. These studies will be continued to examine the effects of speech rate on syllable intelligibility and articulatory accuracy.